1. Field of the Invention
This invention relates to strains of Streptomyces avermitilis lacking branched-chain amino acid transaminase activity and/or branched-chain 2-oxo acid dehydrogenase activity, to methods for producing said S. avermitilis and to the use of S. avermitilis to produce natural and non-natural avermectins.
2. Description of the Prior Art
U.S. Pat. Nos. 4,310,519 and 4,429,042 describe the avermectins, a complex of related agents having potent antiparasitic activity, and their production by aerobic fermentation of strains of Streptomyces avermitilis; namely, S. avermitilis ATCC Nos. 31267, 31271 and 31272. The last two strains cited represent a frozen vial and a lyophilized tube, respectively of a culture obtained by ultraviolet irradiation of S. avermitilis ATCC 31267.
EP 214,731, published Mar. 18, 1987, the counterpart of U.S. patent application Ser. No. 886,867, filed Jul. 16, 1986, discloses a number of compounds (referred to herein as non-natural avermectins) related to the natural or known avermectins but having a novel substituent group at the 25-position, and a process for their preparation by fermentation of an avermectin producing organism in the presence of certain specified carboxylic acids, or derivatives or precursors thereof. The S. avermitilis organisms used to produce the said novel C-25 substituted avermectins are S. avermitilis ATCC 31267, 31271, 31272 and NCIB 12121. The latter organism, described in EP 214,731, is derived from S. avermitilis ATCC 31271. It gives improved yields of the novel C-25 substituted avermectins when it is cultured in a semi-defined medium. Each of ATCC 31267, 31271, 31272 and NCIB 12121 may also produce, in addition to the novel C-25 substituted derivative, varying amounts of the known, or natural, avermectins wherein the 25-substituent is isopropyl or (S)-sec-butyl (1-methylpropyl).
The carbon skeleton of the avermectins (depicted in formula (I) below) is derived from acetates and propionates and the C-25 substituent of natural avermectins from L-isoleucine (R=(S)-sec-butyl) or L-valine (R=isopropyl) [Fisher and Mrozik, "Macrolide Antibiotics", Academic Press (1984) Ch. 14].
By "known" or "natural" avermectins is meant those avermectins produced by S. avermitilis ATCC 31267, ATCC 31271 and ATCC 31272 wherein the 25-position substituent is either isopropyl or (S)-sec-butyl(1-methylpropyl). Avermectins wherein the 25-position substituent is other than isopropyl or sec-butyl (S-form) are referred to herein as novel or non-natural avermectins.
The strains of S. avermitilis cited in the above-mentioned U.S. patents produce a class of substances described generically therein as C-076. The class comprises eight distinct but closely related compounds described as C-076 A1a, A1b, A2a, A2b, B1a, B1b, B2a and B2b. The "a" series of compounds refers to the natural avermectins wherein the 25-substituent is (S)-sec-butyl and the "b" series to those wherein the 25-substituent is isopropyl. The designations "A" and "B" refer to avermectins wherein the 5-substituent is methoxy or hydroxy, respectively. Lastly, the numeral "1" refers to avermectins wherein a double bond is present at the 22-23 position; and numeral "2" to avermectins having a hydrogen at the 22-position and hydroxy at the 23 position.
In this application no such identifiers are used as regards the 25-substituent of the non-natural avermectins. Identifiers A1, A2, B1 and B2 have been retained to refer to non-natural avermectins having the structural features corresponding to those of the natural avermectins as noted above.
Generation of mutants devoid of branched-chain alpha-keto acid dehydrogenase activity has been reported for Bacillus subtilis, Willecke and Pardee, J. Biol. Chem. 246, 5264-72 (1971) and Pseudomonas putida, Martin et al., J. Bacteriology, 115 198-204 (1973), but not for Streptomyces.
S. avermitilis Agly-1, a mutant strain which produces virtually only avermectin aglycones A1a and A2a is reported by Schulman et al. J. Antibiot. 38(11), 1494-1498 (1985). Also reported is the fermentation of S. avermitilis Agly-1 in the presence of sinefungin which caused increased production of avermectin aglycone B components. Likewise, S. avermitilis O8, a high producing strain for avermectins, when fermented in the presence of sinefungin as inhibitor of O-methyl transferases, resulted in production of avermectins lacking O-methyl groups on the aglycone at C-5 and in the oleandrose disaccharide moiety.
U.S. Pat. No. 4,378,353 describes C-076 related compounds and their preparation by cultivation of MA-5218, a mutant strain of S. avermitilis ATCC 31272, obtained therefrom by ultraviolet irradiation. The mutant is identified as ATCC 31780. The C-076 related compounds produced by said mutant lack the C-076 furan ring. Additionally, in certain of the compounds reported, one or both of the oleandrose sugar moieties have been cleaved while in others the 5-position group was oxidized to a keto group.
Three classes of O-methyltransferase mutants of S. avermitilis that produce avermectins lacking O-methyl groups have been reported by Ruby et al., 6th International Symposium on the "Biology of Actinomycetes", Debrecen, Hungary, Aug. 26-30 (1985) and by Schulman et al, Antimicrobial Agents and Chemotherapy 31, 744-7 (1987). The first class produces primarily B avermectins due to their inability to methylate the C-5 hydroxyl of the macrocyclic lactone ring. The second class produces 3'-O, 3"-O-bis-demethylavermectins (avermectins lacking the O-methyl substituent at the 3 position of both oleandrose monosaccharide residues), and which are referred to as demethylavermectins. The third class is unable to methylate at any position.
Schulman et al., Fed. Proc. 44, 931 (1985) disclose increased production of B avermectins by fermenting S. avermitilis in the presence of substances such as sinefungin, S-adenosylethionine and S-adenosylhomocysteine which inhibit the methylation of the C-5 hydroxy group of the aglycone moiety by the enzyme avermectin B-O-methyltransferase. Streptomyces avermitilis mutants which lack O-methyltransferase activity and produce increased amounts of avermectin B components are also disclosed and referred to by Schulman et al. in Antimicrobial Agents and Chemotherapy 29, 620-624 (1986).
Mutagenesis of S. avermitilis produces mutants which lack branched-chain 2-oxo acid dehydrogenase activity or branched-chain amino-acid transaminase activity. Mutagenesis of the thus-produced singly blocked mutants produces mutants which lack both branched-chain 2-oxo acid dehydrogenase activity and branched-chain amino acid transaminase activity. The mutants no longer possess the ability to produce significant amounts of the natural avermectins in the absence of added compound RCOOH wherein R is isopropyl or (S)-sec-butyl, or of a compound convertible to RCOOH during the fermentation process. Surprisingly and unexpectedly, however, the mutants have been found to produce avermectins, natural and non-natural, when fermented in the presence of an added compound R--COOH wherein R is isopropyl or (S)-sec-butyl, or other group disclosed herein, or of a precursor to said RCOOH. It is even more surprising that the herein described mutants which lack only branched-chain 2-oxo acid dehydrogenase activity, and which are unable to degrade L-isoleucine or L-valine, are able to assimilate a wide variety of compounds into the avermectin biosynthetic pathway with production of non-natural avermectins free of the presence of natural avermectins.
At least as surprising is the finding that the herein described branched-chain amino acid transaminase deficient mutants, which are unable to degrade L-isoleucine, L-leucine or L-valine and require these three amino acids in order to grow, are also able to assimilate other compounds to produce non-natural avermectins free of the presence of natural avermectins.
The natural avermectins, as noted, are produced as a complex mixture of eight distinct but closely related compounds; formula (I), R=isopropyl and (S)-sec-butyl. While they have been recovered in substantially pure form (see U.S. Pat. No. 4,429,042), the methodology is, at best, laborious. The production of non-natural avermectins according to the process described in EP 214,731 may also produce some of the natural avermectins in varying amounts due to the presence of the branched-chain 2-oxo acid dehydrogenase and the amino acids L-valine and L-isoleucine in the cell of the S. avermitilis micro-organisms used in their production.
The ability to choose to produce either natural or non-natural avermectins so as to minimize the number and complexity of the products, and by so doing to increase the purity of a chosen avermectin, and thereby to simplify separation procedures, is a desirable goal.